1. Field of the Invention
The present invention relates to a quadrature amplitude modulation (QAM) communication technique, and more specifically, to a scalable QAM communication method, transmitter, and communication system.
2. Description of the Related Art
The QAM modulation technique is frequently implemented in networks with share media, wherein the downlink signals are broadcast to network clients. Examples of such network are digital cable modem, digital television, and 802.16 broadband wireless system. The system service provider broadcasts signals to each network client, however, the signal quality received by each client is different, as the clients are located at different positions and are under different environmental condition. In order to maintain a low uncoded transmission bit error rate, the service provider uses a lower level QAM, for example, 64QAM to modulate the signals. The bandwidth efficiency of the downlink channel is thus restricted by the level of QAM used for modulation.
FIG. 1 shows the topology of the network in the related art which transmits data to shared media, and broadcasts signals over the downlink channel. A transmitter 102 is usually the service provider of the communication system, and the transmitter 102 connects clients 104, 106, and 108 through a network 100. The transmitter 102 broadcasts a QAM modulated signal to the network 100, and the clients 104, 106, and 108 receive the QAM modulated signal sent by the transmitter 102 through the network 100. The signal to noise ratio (SNR) of the signal received by the client 104 is 25 dB, thus the client 104 can only receive and demodulate the 64QAM modulated signal. The SNR of the signal received by the client 106 is 30 dB, enabling the client 106 to receive both 64QAM and 256QAM modulated signals. Furthermore, if the SNR of the signal received by the client 106 is 35 dB, the client 106 can receive 64QAM, 256QAM, and 1024QAM modulated signals as the client 106 is able to demodulate all these signals correctly. The transmitter 102 in this case, however, can only modulate the 64QAM signal in order to allow all the clients 104, 106, and 108 to be able to receive and demodulate the modulated signal correctly.